An eyeball of an animal is not only rotatable vertically and horizontally, but also can rotate about the axis of the visual line direction. The rotation is referred to as “cycloduction”. Cycloduction is generated by the action of trying to return to an original state when tilting the head. In addition, cycloduction is also generated when a person is feeling bad due to vehicle-induced or visually-induced motion sickness, for example, and thus the technique of measuring cycloduction can be useful at medical or nursing care fields.
As a cycloduction measurement method, there has been disclosed (see non-patent literature 1, for example) a method of recognizing a shading pattern of an iris image in an eye image (hereinafter, referred to iris shading pattern), and measuring cycloduction on the basis of the recognition result. In addition, the non-patent literature 1 also describes a technique of suppressing degradation of the measurement precision of cycloduction due to change of the pupil diameter by correcting the deformed iris shading pattern through the use of the fact that deformation of the iris shading pattern is proportional to the pupil diameter.